The present invention relates to the art of diagnostic imaging. It finds particular application in conjunction with rotating one-dimensional (1D) slat-collimated gamma cameras and single photon emission computed tomography (SPECT), and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications and other diagnostic imaging modes such as, e.g., positron emission tomography (PET).
In diagnostic nuclear imaging, one or more radiation detectors are mounted on a movable gantry to view an examination region which receives a subject therein. Typically, one or more radiopharmaceuticals or radioisotopes such as 99mTc or 18F-Fluorodeoxyglucose (FDG) capable of generating emission radiation are introduced into the subject. The radioisotope preferably travels through a portion of the circulating system or accumulates in an organ of interest whose image is to be produced. The detectors scan the subject along a selected path or scanning trajectory and radiation events are detected on each detector.
In a traditional SPECT Anger camera, the detector includes a scintillation crystal that is viewed by an array of photomultiplier tubes. A collimator which includes a grid- or honeycomb-like array of radiation absorbent material is located between the scintillation crystal and the subject to limit the angle of acceptance of radiation which will be received by the scintillation crystal. The relative outputs of the photomultiplier tubes are processed and corrected to generate an output signal indicative of the position and energy of the detected radiation. A detector of this type isolates a scintillation event as originating along an approximate ray or line of view, or more precisely along a narrow-angle cone of view. Because radiation events along a spatial line are projected through an opening of the collimator array grid or honeycomb, the collected data is often referred to as projection data. The projection data is then reconstructed into a three-dimensional image of a region of interest by a reconstruction processor.
A rotating laminar emission camera, also known as the rotating laminar radionuclide camera, has linear collimators usually formed by mounting parallel collimating plates or slats between a line of individual detectors. Alternately, individual detector areas of a large-area detector are defined and isolated through the placement of slats. The slat collimator isolates planar spatial projections; whereas, the grid collimator of traditional scintillation detectors isolates essentially linear spatial projections. The detector-collimator assembly of a slat camera is typically rotated about an axis perpendicular to the detector face in order to resolve data for accurate two-dimensional image projection. Again, projection data collected at angular orientations around the subject are reconstructed into a three-dimensional volume image representation.
While maintaining certain advantages, such as a better sensitivity-resolution compromise, over, e.g., traditional Anger cameras, slat detectors are burdened by some other undesirable limitations. For example, the one dimensional collimation or slat geometry used by slat detectors complicates the image reconstruction process. The slat geometry results in a plane integral reconstruction as opposed to the line integral reconstruction that is generally encountered in traditional Anger camera applications. Moreover, the geometry produces a plane integral only in a first approximation.
In actuality, the plane integral should have a weighting factor introduced thereto to account for the fact that the detector""s sensitivity has a 1/r dependence, where r represents the distance between a detected radiation event occurring in the object under consideration and the detection point on the detector. That is to say, the detector is generally more sensitive to relatively close objects and less sensitive to far away objects.
Reconstruction of linear projection data obtained using conventional Anger cameras usually incorporates backprojection using a form of the inverse Radon transform Rxe2x88x921. Reconstruction of the planar projection data obtained from a slat-type camera is complicated in two respects. First, the integrations are planar integrations rather than line intregrals. Second, the 1/r term which occurs in projection data obtained by a slat detector reduces the spatial symmetry of the projection data. The reduced symmetry prevents the use of mathematical methods which are typically employed to implement the Radon transform R and its inverse Rxe2x88x921.
Most previous reconstruction methods for projection data acquired by a slat detector merely disregard or ignore the 1/r weighting factor in solving the reconstruction problem. This approximation results in degradation of the reconstructed image. This type of image degradation could be reduced or even eliminated by a new or improved reconstruction algorithm which accounts for the 1/r dependence.
The present invention contemplates a new and improved reconstruction technique which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a nuclear medical imaging apparatus is disclosed. An object is received in a receiving region. A radiation detector has a side facing the receiving region. The detector includes a collimator fabricated from radiation attenuative material arranged on the detector side facing the receiving region. The collimator includes a plurality of spaced apart slats. The detector also includes an essentially linear array of detecting elements, the detecting elements being disposed between the slats on the detector side facing the receiving region. The imaging apparatus further includes an image reconstruction processor which converts the projection data from the detector into an image representation. The image reconstruction processor includes a memory, a preconditioning operator P, a projection operator S, and an iterative loop operator which applies the preconditioning operator P and the projection operator S to the memory contents to calculate updated memory contents. Preferably, the preconditioning operator P applies an inverse Radon transform operator Rxe2x88x921. In one embodiment, the memory stores projection data, and the iterative loop operator applies the preconditioning operator P to the projection data stored in the memory, and then applies the projection operator S to produce a second set of projection data. The projection operator preferably incorporates a plurality of Radon transforms R, each Radon transform being applied to an image weighted by a weighting factor selected such that the projection operator approximates the projection transform physically implemented by the radiation detector, the approximating including at least approximating a 1/r dependence of the projection data generated by the radiation detector.
In accordance with another aspect of the present invention, a diagnostic imaging system is disclosed. A scanner generates projection data that is weighted inversely with distance in a projection direction. A backprojector backprojects the generated projection data into an image memory without compensating for the inverse weighting with distance to reconstruct an artifacted image representation. A forward projector forward projects the artifacted image to generate reprojected data. A correction circuit (i) compares the generated projection data and the reprojection data, and (ii) generates a correction factor in accordance with a deviation between the generated projection data and the reprojection data. The scanner preferably includes a one-dimensional array of radiation detectors, a collimator which collimates received radiation into planes, and a rotor for rotating radiation planes around an axis perpendicular to a face of the detector array.
In accordance with another aspect of the present invention, an image reconstruction process for generating a final image from measured projection data acquired by a slat detector is disclosed. The measured projection data is preconditioned using a preconditioning operator P to obtain an image. The image is iteratively improved to obtain a final image. The iterative process includes projecting the image with a projection operator S to generate reprojected data, comparing the reprojected data with the measured projection to generate correction data, and backprojecting one of the correction data, the reprojection data corrected with the correction data, and the projection data corrected with the correction data, using the preconditioning operator P to obtain an improved image. Preferably, the preconditioning operator P incorporates an inverse Radon transform Rxe2x88x921.
In accordance with yet another aspect of the present invention, a diagnostic imaging process is disclosed. Projection data is generated which is weighted inversely with distance in a projection direction. The projection data is backprojected into an image representation without compensating for the inverse weighting to reconstruct a flawed image representation. The flawed image representation is forward projected to form reprojected data. The reprojected data is compared with the projection data. A correction is generated from a deviation between the reprojection data and the projection data. A backprojecting of one of: (i) the correction data into the flawed image representation; and (ii) the correction data combined with one of the projection and reprojection data is performed to generate a less flawed image representation. Steps starting with the forward projecting are iteratively repeated until the comparing step meets a preselected closeness criteria. Preferably, the backprojecting step uses an inverse Radon transform. The projection data generating step preferably includes: introducing a radiation source into a subject; collimating radiation from the source into planes; detecting the radiation in each plane; and rotating the collimation planes around a first axis parallel to at least one of the planes. Additionally, the data generating step may include rotating the collimation planes about a second axis through the subject and transverse to the first axis.
One advantage of the present invention is that it corrects for the 1/r dependence of the data.
Another advantage of the present invention is that it properly reconstructs data obtained from slat cameras.
Another advantage of the present invention is that it efficiently transforms the measured projection data into image space without neglecting the 1/r dependence of the slat detector projection data.
Yet another advantage of the present invention is that it utilizes the Radon transform R, which is often implemented in reconstruction algorithms for SPECT, PET, and other nuclear imaging methods, as a preconditioner.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description.